A stereomicroscope permits observation with the same three-dimensional comprehension as that provided by both eyes while a solid object is being observed. Therefore, it is easy to obtain a perspective relation between a tool, such as tweezers, and an object while working in an image field of a stereomicroscope. Thus, a stereomicroscope is especially effective in applications requiring minute manipulation such as precision machining, dissection in biological research and surgical operations. To obtain parallax for three-dimensional comprehension with a stereomicroscope, portions of the optical elements through which light rays pass to enter into both right and left eyes of the observer are independent of each other. The two rays which enter both eyes are made to intersect in the plane of the object being observed. The enlarged images viewed from different directions are formed and are observed through the eyepieces of the microscope, allowing stereovision of a small object under observation. This type of stereomicroscope is known as "Greenough-type" stereomicroscope. The Greenough-type stereomicroscope is one of the standard stereomicroscopes used for obtaining a stereovision of an object.
FIG. 4 is a schematic diagram of a Greenough-type stereomicroscope SM1 according to related art. Stereomicroscope SM1 comprises an optical system OSR for the right eye and the optical system OSI for the left eye. The respective optical axes AX1 and AX2 of optical systems OSL and OSR are arranged at a predetermined angle .theta. and intersect at optical axis AX0 bisecting optical axes AX1 and AX2. An image of an object 1 arranged at the intersection of optical axes AX1 and AX2 is formed at positions 7 and 7' by imaging lenses (usually zoom lens) 6 and 6' having a magnification .beta.. The magnification B of optical systems OSL and OSR from object 1 to images 7 and 7' is .beta.. Images 7 and 7' are then magnified respectively by eyepiece lenses 8 and 8', and are observed by the naked eyes of an operator (not shown) of the microscope placed at eyepoint positions 9 and 9' respectively.
In a Greenough-type stereomicroscope, for an individual to observe an object in a free and comfortable posture, it is necessary to change the position of eye-points 9 and 9' of the eyepiece along respective optical axes AX1 and AX2, to the eye-level of the observer, which in part depends on the height of the observer. To raise the eye-level, the distance between object 1 and eye-points 9 and 9' needs to increase. On the other hand, to lower the eye-level, the distance between object 1 and eye-points 9 and 9' needs to decrease. In a Greenough-type stereomicroscope, use of an auxiliary objective lens is a known method for changing the eye-level. FIGS. 5 and 6 demonstrate the known method of changing eye-level using an auxiliary objective lens. The stereomicroscopes in FIGS. 5 and 6 are constructed the same as that in FIG. 4 and the same reference numbers are used to identify parts equivalent to those in FIG. 4.
FIG. 5 provides a schematic diagram of the optical system using, the currently known method for raising the eye-level of a Greenougih-type stereomicroscope. As depicted in FIG. 5, by inserting an auxiliary objective lens 14 (shown as a negative lens) between object 1 and imaging lenses 6 and 6', the distance from object 1 to the imaging lenses is extended and the eye-level is raised. In FIG. 5, P1 denotes the position of object 1 when auxiliary objective lens 14 is not inserted, and P3 denotes the position of the object when the auxiliary objective lens is inserted. Also, D41 denotes the axial distance between object 1 at position P1 and a principal plane 14h (principal point) when auxiliary objective lens 14 is not inserted. D43 denotes the distance between position P3 of object 1 and principal plane 14h (principal point) when auxiliary objective lens 14 is inserted. D13 denotes the distance between position P1 of object 1 when auxiliary objective lens 14 is not inserted, and position of P3 when auxiliary objective lens 14 is inserted. Since auxiliary objective lens 14 is inserted between object 1 and imaging lenses 6 and 6', the distance from the object to imaging lenses 6 and 6' is extended by moving the position of the object observed from position P1 to P3 , raising the eye-level. In this configuration, the eye-level is raised by the amount D13 by inserting auxiliary objective lens 14. Position P1 and P3 are along optical axis AX0.
The magnification .beta.a of auxiliary objective lens 14 is determined by the following equation: EQU .beta.a=(D41/D43)&lt;1.
The magnification B from object 1 at position P3 to images 7 and 7' is given as: EQU B=.beta.a.times..beta..
FIG. 6 is a schematic diagram showing the currently known optical system used in stereomicroscope SM1 for lowering the eye-level. As depicted in FIG. 6, an auxiliary objective lens 15 (shown as positive lens) is inserted between object 1 and imaging lenses 6 and 6'. This decreases the distance from object 1 to imaging lenses 6 and 6', thus lowering the eye-level. In FIG. 6, P1 denotes the position of object 1 when auxiliary objective lens 15 is not inserted, and P3 denotes the position of the object when auxiliary objective lens 15 is inserted. D41 denotes the distance between object 1 at position P1 and a principal plane 15h (principal point) when auxiliary objective lens 15 is not inserted. D43 denotes the distance between object 1 at position P3 and principal plane 15h (principal point) when auxiliary objective lens 15 is inserted. D31 denotes the distance between the position P1 of object 1 when the auxiliary objective lens 15 is not inserted and position P3 of the object when auxiliary objective lens 15 is inserted. When auxiliary objective lens 15 (positive lens) is inserted between object and imaging lenses 6 and 6', the distance from the object to the imaging lenses is decreased, lowering, the eye-level. In this configuration, the eye-level can be lowered by the amount of D31 by inserting auxiliary objective lens 15.
The magnification .beta.a of auxiliary objective lens 15 is determined by the following equation: EQU .beta.a=(D41/D43)&gt;1.
The magnification B from the object 3 to the images 7 and 7' is given by: EQU B=.beta.a.times..beta..
As is described above, according to related art, to adjust the eye-level either up or down to the height of an observer looking through the Greenough-type stereomicroscope, one must change the auxiliary objective lens each time. This is cumbersome, prevents quick observations in different positions, and disrupts careful observations, as changing the auxiliary lenses can jar the object under observation.